1. Field of the Invention
This invention relates to polarizing broad bands of light and particularly to polarizing broad bands of light at large angles of incidence from any azimuth without polarization or color distortion in both transmission and reflection modes.
2. Description of the Related Art
Cholesteric Liquid Crystals (CLC) have the property of being able to reflect or transmit circularly polarized light depending on the handedness of the light, reflecting one circular polarization and transmitting the opposite circular polarization. For example, a right handed cholesteric liquid crystal will reflect right handed circularly polarized light and transmit left handed circularly polarized light. Cholesteric liquid crystals can be made which function over a large bandwidth such that all the colors in the spectrum can be reflected or transmitted. These properties are very useful in many applications and work well when the incident light is normal to the cholesteric liquid crystal films. However, polarization and spectral distortions occur at large angles of incidence and vary at different viewing angles due to the eigen-states of the light being elliptical rather than circular. These distortions reduce the polarizer efficiency and produce color shifts. Many attempts to correct these polarization and color distortions have been made in the past.
For example in U.S. Pat. No. 5,731,886 entitled xe2x80x9cBirefringent Compensator for Reflective Polarizersxe2x80x9d, Taber et al. issued Mar. 24, 1998 the invention comprises a CLC material layer and a compensator comprising a positively birefringent C-plate layer which is a uniaxial film with an optical axis perpendicular to the surfaces of the film. The compensation film is inserted on the path of the light transmitted through the CLC polarizer, and incorporated into a brightness enhancement system for a liquid crystal display.
The simulation results presented in the patent indicate an improvement in color and luminance behavior of the LCD brightness enhancement system when the C-plate is incorporated into the system. However, no spectra of the CLC polarizers used in these simulations are presented, either with or without an included compensator. The only information about the CLC polarizers is their end-point values of the pitch (0.26 and 0.402 microns) which suggest a reflection band from about 400 nm to 600-650 nm. Improvement in chromaticity behavior of the brightness enhancement system does not necessarily indicate that the CLC polarizer angular behavior has been completely compensated. In addition, the chromaticity of the system is presented for only one azimuthal orientation, and need not be the same at all azimuthal orientations. This compensation technique improves the polar angular behavior of the chromaticity at one azimuthal orientation only. European Patent Application EP 0860717 A2 published Aug. 26, 1998 entitled xe2x80x9cBroadband Cholesteric Optical Device, Polarizer, Filter, Liquid Crystal Device, and Polarizing Beam-Splitterxe2x80x9d, proposes to improve the viewing angle behavior of the broadband CLC-based polarizers by using compensation films, which are uniaxial and have their optical axes perpendicular to the surfaces. The compensation films typically consist of two layers. The layer closest to the CLC polarizer has a positive birefringence, and the second layer has a negative birefringence. These two-layer compensation films may be inserted either in front of the CLC polarizer, or at the back, or both. This compensation technique requires dispersion of the refractive indices of the compensation films. The patent application gives formulas indicating the necessary dispersion relationships.
The CLC polarizer used in the simulations described in the patent has a bandwidth from about 380 nm to 700 nm, and thus would not cover the entire visible range for all incident angles. As the patent indicates, the proposed technique is capable of improving the angular performance of this CLC polarizer up to incident angles of 49 degrees. In order to achieve the proposed compensation, dispersion of the refractive indices of both compensation films is a prerequisite. The authors have derived formulas to specify the required dispersion behavior. These requirements may be quite difficult to satisfy in practice.
Impractical material requirements, however, are not the major drawback of this compensation method. The CLC polarizer spectra presented in the patent indicate that even at normal incidence the polarization state that has to be reflected from the polarizer is actually 20-30% transmitted. When unpolarized light is incident on such a polarizer, saturated reflection will not be achieved, that is, more than 50% of the incident light will be transmitted, and very importantly, the transmitted light will be only partially polarized. This feature will severely degrade the contrast ratio of any system utilizing this CLC reflective polarizer.
A very important aspect of this compensation technique is that it actually would be ineffective if the polarizers were designed so that a saturated reflection did occur for the entire visible range at all incident angles (that is, if the polarizers were thicker, with higher birefringence, and larger bandwidth).
European Patent Application EP 0860716 A2 published Sep. 26, 1998 entitled xe2x80x9cBroadband Cholesteric Optical Device, Polarizer, Filter, Liquid Crystal Device, and Polarizing Beam-Splitterxe2x80x9d discloses a multi-layer system to serve as a broadband reflective polarizer with improved viewing angle performance. The system consists of alternating narrow-band CLC and homeotropic films with positive birefringence. Broadband reflection is achieved by requiring each CLC layer to have a different peak reflection wavelength. The multi-layer design is to achieve a reflective circular polarizer with superior angular performance. The patent application presents simulation results for a 15-layer system, which is capable of reflecting in the range from 380 nm to 630 nm at normal incidence. The angular behavior of such a polarizer is improved for incident angles of up to 40 degrees.
To produce a reflective circular polarizer with a larger bandwidth and superior angular performance would require the addition of more CLC and homeotropic layers, all having precisely controlled parameters. These requirements make the fabrication of such a system difficult, prone to defects, and expensive.
U.S. Pat. No. 5,808,794 entitled xe2x80x9cReflective Polarizers Having Extended Red Band Edge for Controlled Off-Axis Colorxe2x80x9d, issued Sep. 15, 1998 attempts to solve the angular behavior problem of the broadband polarizers, with a broadband reflective linear polarizer consisting of alternating birefringent and isotropic layers with variable thickness. This linear polarizer does not have polarization distortions at large viewing angles. However, in order to achieve a reflection band covering the entire visible range at all incident angles, it must have at least 800 layers with precisely controlled layer thickness. This complicated structure makes the manufacturing procedure prone to defects and expensive. In addition, the layers have been found to delaminate at extreme environmental conditions, which makes the polarizer inappropriate for military and avionics applications.
A broadband polarizer and analyzer with two compensation films to compensate the polarization distortions of the light at large angles caused in the polarizer and analyzer layer. The compensation layers bring the light back to circularly polarized light without distortion of chromaticity for all incident angles at all azimuthal orientations.
The broadband polarizer and analyzer is typically a broadband CLC film having a variable pitch helix. Light incident normal to the CLC layer is reflected or transmitted as circularly polarized light with no distortion. Light incident at small angles to normal will have very small distortions. However, light incident at larger angles has more distortions and needs to be corrected for. The first layer of compensation film is a CLC infrared layer to rotate the major axis of the polarization ellipse to xc2x145 degrees. The second compensation film is a homeotropic layer to convert the light to circular light.
The CLC film with only two compensation layers provides good polarization with no color distortion for incident angles up to about 70 degrees from normal.
The device can be used to analyze light in transmission mode and reflection mode. Similarly, the device can be used to polarize light in transmission mode or reflection mode.
It is an object of the invention to polarize unpolarized incident light in reflection mode for a large bandwidth and a wide range of incident angles.
It is an object of the invention to polarize unpolarized incident light in transmission mode for a wide range of angles and a large bandwidth.
It is an object of the invention to analyze circularly polarized light for a wide range of angles and a large bandwidth in reflection mode.
It is an object of the invention to analyze circularly polarized incident light in transmission mode for a wide range of angles and a large bandwidth.
It is an object of the invention to transmit broadband polarized light without spectral distortions for a large range of angles.
It is an object of the invention to compensate for the color change associated with using reflective CLC polarizers.
It is an object of the invention to compensate for elliptical distortions of circularly polarized light in cholesteric liquid crystals when incident light is at large angles.
It is an object of the invention to compensate for spectral distortions of circularly polarized light in cholesteric liquid crystals when incident light is at large angles.
It is an object of the invention to compensate for elliptical distortions of circularly polarized light in cholesteric liquid crystals when the light is viewed at large viewing angles.
It is an object of the invention to compensate for color distortions associated with polarization distortions of circularly polarized light in cholesteric liquid crystals when the light is viewed at large viewing angles.
It is an object of the invention to compensate the severe degradation in polarization behavior at large incident angles associated with CLC-based broadband polarizers.
It is an object of the invention to obtain a 45-degree circularly polarizing beam splitter.
It is an object of the invention to provide polarizing beam splitters.
It is an object of the invention to provide high-efficiency passive smart windows.
It is an object of the invention to provide high-efficiency switchable smart windows.
It is an object of the invention to provide polarized light sources with 100% polarization conversion efficiency.
It is an object of the invention to provide polarized beam combiners.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.